Note: Descriptions are shown in the official language in which they were submitted.
58732
This invention relates to the improvement of light
emission efficiency of light emitting diodes (LED`s) and
particularly in relation to coupling to optical fibres.
In semiconductor LED's, with light being emitted in
a direction normal to the plane of the active layer or light
emitting layer, the vast majo~ity of the light produced is lost
by internal absorption. Thus, for example, with GaAlAs material,
the critical angle of emission is about 17 from the axis normal
to the light emitting layer. In a device having parallel faces,
any rays encountering the upper surface at an angle greater than
this critical angle are reflected back into the device. There-
after, the rays will always encounter the surface at the same
angle and will never exit, being finally absorbed.
The present invention relates to the preparation
of the outer surface of the device, at the light emittlng region,
such that a ray encountering this surface at an angle greater
than the critical angle is reflected but is capable of encountering
this outer surface, after one or more reflections, at an angle
less than the critical angle. Improvements in efficiency between
50% and 150% have been achieved.-
In a particular application of the invention,the surface of the device is roughened, as by etching.
In its broadest aspect the invention comprises
a diode having a First confining layer of GaAlAs, an active layer
of GaAs and a second confining layer of GaAlAs in a sandwich
formation, the active layer between the confining layers, a light
emission surface on one of the confining layers, the surface of
random roughness, and means for applying a voltage bias to the
diode to produce a light emitting area aligned with the light
3~ emission surface, the means for applying the bias including a
metal contact layer on the surface of the other confining layer,
1~58732
the metal contact layer forming a mirror aligned with the light
emitting area.
Particularly the layers are on a substrate with
an aperture therethrough to the one conFining layer, the l;ght
emission surface aligned with the aperture, and said metal contac~
layer is formed over an bptically transparent oxide insulating
layer, the metal contact layer in contact with the other confining
layer through an aperture in the oxide layer and aligned with
the aperture in the substrate.
The invention will be readily understood by
the following description in conjunction with the accompanying
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diagrammatic drawings, in which:-
Figure 1 is a cross-section through an LED
illustrating the critical angle,
Figure 2 is a cross-section through an LED,
similar to that in Figure 1, but illustrating a roughened surface
at the light emission surface;
Figure 3 is a cross-section through a coupling
structure for coupling an optical fibre to an LED.
As illustrated in Figure 1, an LED structure 10
has a light emitting volume or layer 11. The surface through
which light emits or issues is indicated at 12. A ray of light
13 emitted from the layer 11 normal to the plane of the layer 11,
and the surface 12, will issue from the device. A ray 14 emitted
at an angle ~ will also issue from the device - assuming ~ to be
the critical angle, as will any ray between these two rays 13 and
14, for example ray 15. Any ray emitted from layer 11 at an angle
greater than ~, for example ray 16, will be internally reflected
and will never meet the surface 12 at a different angle, however
3~ many times it is reflected. It is eventually internally absorbed.
Figure 2 illustrates a device or structure 20,
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having the light emitting volume or layer ll and a surface 21
through which light emits or issues. The surface 21 is roughened
to give a non-regular or haphazzard profile, viewed in cross-
section. By this means~ a ray which would not issue through a
flat service can possibly encounter the surface at an angle less
than the critical angle - as for example ray 23. A ray encountering
the surface 12 at an angle greater than the critical angle is
reflected to the rear surface 24 and then reflected to the surface
21 and this time can possibly encounter the surface at an angle
less than the crit1cal angle - for example as ray 25. The
roughness of surface 21 is shown exaggerated to illustrate the
basic feature of the invention.
Figure 3 illustrates in more detail, a particular
embodiment of the invention, in relation to the coupling of the
light emitted into an optical fibre. A substrate 30 of semi-
conductor material, in the present example GaAs, has a plurality
of layers formed thereon, as by epitaxial growth, the layers in
sequence from the substrate 30 being a first confining layer 31
of GaAlAs, an active layer 32 of GaAs and a second confining layer
33 of GaAlAs. The conductivity types are such that the substrate
30 and first con~ining layer 31 are the same type, the active
layer 32 can be the same or opposite type as the substrate and
first confining layer, and the second confining layer is of the
opposite conductivity type to that of the substrate and First
confining layer. Thus, for example, substrate 30 and first
confining layer 31 are of n-type, active layer 32 is n-or p-type,
and the second confining layer 33 is p-type. The two confining
layers 31 and 33 and the active layer 32 are doped to suitable
levels, as is well known. For example the confining layers can
3~ be doped to a level of 1018, while the active layer 32 can be `~
doped to a level of, for example between 1017 and 1018. The
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confin;ng layers 31 and 33 contain aluminum to a predeterminedvalue while the active layer 32 may or may not include aluminum.
If aluminum is included it is at a significantly lower level than
that of the aluminum contact of the confining layers - as is
conventional.
After formation of the layers 31, 3Z and 33, the
substrate 30 is masked and etched - typically by photolithographic
etching techniques - to produce a hole or aperture 3~ through the
substrate down to the sur~ace of layer 31. A typical etchant is
25 parts of H202 to 1 part of NH40H. A layer of metal oxide 35
is then formed on the surface of layer 33. An aperture 36 is
formed in the oxide layer 35 axially aligned with the hole 34 in
the substrate. The aperture 36 can be formed by masking before
forming the layer 35, or by forming the layer 35 right across the
surface of layer 33, and then photolithographically etching the
aperture. A metal contact layer 37 is then formed over the oxide
layer and extending into the aperture 36 and into contact with the
layer 33. Also a contact layer 38 is formed on the substrate 30.
Finally, the surface 39 of the layer 31 exposed at the bottom of
the hole 34 is roughened, by etching, possible etchants including
30/10/3(CH3COOH - HN03 - HF) for forty seconds at 25C. This
etchant does not attack gold contacts on the device. Another
etchant is KI/I2 for thirty seconds at 60C, but this will attack
gold contacts.
As a result of current spreading in layers 31, 32
and 33, the emitting area 40 is larger than the aperture 36 by an
annulus 41. The combination of metal layer 37 cGvering the oxide
layer 35 which surrounds the aperture 36 forms a highly effective
mirror for light which is emitted by the emitting area 40. In ;
particular, a highly efficient mirror exists in the annular region
44, for light which is emitted at the annular emitting region 41,
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normal or near normal to the plane of the emitting area 40. The
metal contact layer 37 in direct contact with layer 33 also acts
as a partial mirror, reFlecting rays which encounter it back
to wards the surface 39.
The emitting area 40 is designed to be smaller than
the hole 34. The core 42 of the optical fibre 43 is positioned `
opposite to the emitting area 40. In general, the core of the
fibre is larger than the emitting area 40.
A particularly useful ratio of emitter diameter to
fibre core diameter is 60/80, a typical size being a 60 ~m emitter
region 40 and a 80 ~m diameter for the light transmitting fibre
core 42. However the invention is applicable to various fibre
diameters, and fibres with diameters of 125 ~m and 52 ~m have
been used. The numerical apertures of such fibres can be of
differing values also.
Typical results for a number of devices have
shown efficiency increases of up to 150%. It is believed that
this large increase in efficiency can be attributed to the
comb;nations of the roughened emission surface and the use of a
reflecting surface at the back of the device. Hitherto, it has
been considered that, in contrast with the situation which exists
LED's which emit visible light, i.e. those made from GaP, the
absorption characteristics of GaAlAs are such that most light
emission from a device occurs at the first encounter between
directly emitted rays and the emission surfaces, i.e. rays `;
emitted toward the emission surface, and that any rays reflected
back into the device woùld be absorbed. Any rays emitted rear- -
wardly, that is away from the emission surface, were assumed to -
be absorbed before they would be reflected back toward the
3~ emission surface, or would be absorbed aFter such reflection but
before reaching the emission surface.
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73;2
However, the provision of the reflecting surface,
formed by the metal/metal oxide interface, has been found to
increase emission, and that rays emitting in directions relative -
to the emission surface, other than at or below the critical angle,
can be reflected back to the em;ssion surface. Also rays -
reflected from the emission surface can "bounce" back and such
paths can indicate a number of `'bounces". The rough emission
surface increases the likelihood that such reflected rays will
encounter the emission surface at an angle at - or less than the
10 critical angle.
To obtain high coupling efficiency into the fibres
42, an index matching fluid can be positioned in the aperture
34, between the end of the fibre 42 and the sur-face 39. e
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